1. Field of the Invention
The invention relates to thin-film thermoplastic bag structures and, more particularly, is concerned with handled bag structures useful to transport goods such as groceries.
2. Brief Description of the Prior Art
The bag art is an old one and more recently has evolved from bags made of paper and cloth to flexible bags made of extruded films of a synthetic polymeric resin. An early U.S. Pat. (No. 1,253,269) issued to W. J. Moeller in 1918 described paper and cloth bags of a triangular, truncated construction. Subsequently, the U.S. Pat. No. 2,688,914 (Eckler) described envelopes for containing slices of citrus fruits, made of heat-sealable thermoplastic resins in the form of a flat trapezoidiform sack.
The use of flexible films of thermoplastic resins to fabricate bags and bag-like containers enabled the art to make containers in a variety of shapes including trapezoidal; see for example U.S. Pat. No. 3,003,681 issued in 1961 to Orsini.
Further improvements included the attachment of handles to the flexible bags; see U.S. Pat. No. 4,040,562 (Ward, et al.), also describing a flat, frusto-conical shaped bag.
In more recent years, the art has fastened its attention on development of tubular film bags having handles and suitable for use in carrying, for example, groceries from grocery markets. The special requirements of the grocery bag are high-tensile strength, impact and penetration resistance, and handle strength, the bag being tear resistant at stress points. Further requirements from the manufacturers' view point are extremely thin, flexible films having a minimum resin weight to promote low cost. Representative of the latter bags are those described in U.S. Pat. Nos. 4,165,832 (Kuklies, et al.) and 4,326,664 (Benoit, et al.). These bags, known in the art as "T-shirt bags" because of their handled shape, are prepared by blow extruding a film of resin in a tubular shape, folding the tube in a certain manner, and cutting continuous lengths of the extruded tube into appropriate sizes. The sides and ends may be heat sealed to close the bag structure. Techniques of extruding the films are also well known in the art; see for example U.S. Pat. Nos. 4,022,558; 4,118,453; 4,472,343; and 4,626,397.
Present commercially available T-shirt bags include straight gussets. These bags have inherent weak points at the middle zone of the bag bottom where the gussets are sealed to the base. The bags of the present invention have varying gusset depths (widths) below the handle area. This particular construction shifts the inherent weak points to the outer periphery of the bags where the loads are not usually as severe as they are at the base center of the bag. The invention is particularly useful in the construction of bags from polyolefin resins. In the prior art, polyolefin bags, particularly those made from high molecular weight-high density polyethylenes, are prone to develop holes when the bags are loaded and lifted. Even with linear low density polyethylene bags, the seal of the gussets at the base, having the same stress points, develops holes during use. This problem is substantially obviated by the particular construction of the bags of the present invention.